1. Field of the Invention
The present invention relates generally to inkjet printheads, and more particularly to a method for compensating shift in ON resistance of transistors of the inkjet printheads.
2. Description of the Related Art
Inkjet printers print data on a physical print medium, such as a paper sheet, a transparency and the like, by discharging ink droplets thereon. An inkjet printer includes one or more ink tank for storing ink therein, and a fluid ejection head (herein after referred to as printhead) coupled to the ink tank for discharging ink droplets therefrom. The ink tank may contain one or more ink reservoirs for storing the ink therein. The one or more ink reservoirs may contain colored ink or mono-chrome ink.
The printhead may be disposed on an ink tank body, thereby configuring a print cartridge. When the ink contained in the one or more ink tanks (herein after referred to as the ink tanks) is exhausted, the print cartridge may be replaced with a new print cartridge. Such a printhead configured on a replaceable print cartridge is referred to as a “disposable printhead.” Alternatively, the printhead may be permanently disposed on the printer, and in such an instance only the ink tank is replaceable. Such a non replaceable printhead is referred to as a “permanent printhead.” However, irrespective of the type of printhead being used in the inkjet printer, the printhead is always in operative coupling with the ink tanks for receiving the ink therefrom.
Referring to FIG. 1, a cross sectional view of a prior art printhead 100 is illustrated. Printhead 100 includes a printhead chip 105 having a semiconductor substrate 110 (herein after referred to as substrate 110) and a plurality of heaters, such as a heater 115, bonded to substrate 110. Substrate 110 is operatively coupled to an ink tank (not shown), and includes a plurality of openings, for example an opening 120, for supplying ink therethrough to a plurality of bubble chambers, such as a bubble chamber 125, configured in printhead 100. Bubble chamber 125 acts as a temporary ink tank for receiving the ink from the ink tanks and storing the same therewithin. Moreover, printhead chip 105 includes a memory module (not shown) adapted to store parameters associated with the inkjet printer, such as a value of a number of pages printed by the printhead, a value of a number of pages printed by an ink tank operatively coupled to the printhead, an identification number of the ink tank operatively coupled to the printhead, and the like. However, it will be evident to a person skilled in the art that the parameters may also be stored in a memory associated with the inkjet printer.
The printhead 100 further includes a nozzle plate 130 having a plurality of nozzles, for example, a nozzle 135 for dispensing ink therefrom. Nozzle plate 130 may be bonded to substrate 110 by any conventional technique, for example, by forming an adhesive layer therebetween. More specifically, nozzle plate 130 is bonded to substrate 110, such that the plurality of heaters is disposed beneath the plurality of nozzles of nozzle plate 130. For example, heater 115 may be disposed beneath nozzle 135. Moreover, substrate 110 bonded to nozzle plate 130 configures a plurality of bubble chambers between the plurality of heaters and corresponding nozzles, and a channel 140 for supplying the ink therethrough to the plurality of bubble chambers. More particularly, the plurality of bubble chambers are open spaces above each of the heaters of the plurality of heaters, which are adapted to receive the ink from the one or more ink tanks. As shown in FIG. 1, bubble chamber 125 is configured over heater 115.
In operation, the ink supplied by the ink tank flows into bubble chamber 125 through opening 120 and channel 140. Based on an image to be printed on the print medium, the printer selectively activates some of the heaters of the plurality of heaters. The activated heaters (for example, heater 115) in turn heat the ink received in the corresponding bubble chambers (for example, bubble chamber 125). As a result, an expanding bubble is formed within the ink. The expanding bubble expels the ink through the nozzles corresponding to the activated heaters, onto the print medium, thereby forming small dots of ink on the print medium. Accordingly, a selective activation of the heaters of the printhead may be utilized to print required data on the print medium.
The selective activation of the plurality of the heaters may be controlled by means of a control circuit. More specifically, the control circuit is operatively coupled to the plurality of heaters for controlling an activation of the plurality of heaters. The control circuit includes a plurality of transistors. Examples of the plurality of transistors of may include, but is not limited to a Power Field Effect Transistor (FET). The plurality of transistors is configured on printhead chip 105.
During operation of the plurality of heaters, individual heaters, such as heater 115, may be activated for heating the ink contained in a corresponding bubble chamber, such as bubble chamber 125. The firing of individual heaters may be controlled by corresponding transistors of the control circuit. With reference to FIG. 2A, a control circuit 200 for controlling activation of a heater, such as a heater 205 is illustrated. Heater 205 is similar to heater 115, and is connected to a power source, for example, a battery (represented as ‘+V’).
Control circuit 200 may be disposed on a conventional printhead chip (not shown), similar to printhead chip 105. Moreover, control circuit 200 includes an FET 210 operatively coupled to heater 205. More specifically, a drain terminal 210a of FET 210 is electrically coupled to heater 205, while a source terminal 210b of FET 210 is electrically coupled to a ground terminal 215. Moreover, FET 210 includes a gate terminal 210c such that on applying an appropriate gate voltage at gate terminal 210c, the operation of FET 210 may be controlled. More specifically, FET 210 may be driven to a conduction stage or out of conduction stage by applying the appropriate gate voltage.
Control circuit 200 further includes a gate drive circuit 220 adapted to control the gate voltage input to gate terminal 210c of FET 210. Accordingly, control circuit 200 is adapted to drive FET 210 to the conduction stage or out of the conduction stage. Preferably, gate drive circuit 220 may generate a plurality of electrical pulses, hereinafter referred to as “fire instructions”, which is supplied to gate terminal 210c for driving FET 210 to conduction. Once FET 210 is driven to the conduction stage, a conducting path is configured between the power source “+V” and ground terminal 215, thereby causing a current to flow from the power source “+V” to ground terminal 215 through heater 205 and FET 210. The current flowing through heater 205 effectuates heating thereof, which is utilized to heat the ink contained in a corresponding bubble chamber of heater 205.
Practically, it has been observed that efficiency of the heater and FET arrangement, such as heater 205 and FET 210, of the printhead reduces over a life span of the printhead. More specifically, a heating effect produced by the heaters may reduce during continued usage of the printhead, thereby deteriorating quality of images printed by the printhead. The reduction in the heating effect of the heaters, such as heater 205, may be attributed to a shift in a conduction state resistance or commonly referred to as ON resistance (hereinafter referred to as “RON”) of the corresponding transistors, such as FET 210, in a chip of the printhead. More specifically, the RON of the FETs increases over the life span of the printhead, thereby increasing a power loss in the transistors, such as FET 210, and consequently decreasing the heating effect produced by the heaters, such as heater 205.
Various experimental results have shown that high operating voltages of the heaters and high firing frequencies of the printheads results in an increase in a value of the RON of the transistors in the chip of the printhead. With reference to FIG. 2B, a curve 250 depicting an increase in the value of RON of a transistor, for example FET 210, in a chip of a printhead over a life span of a printhead is illustrated. The life span of the printhead is hereinafter referred to as “life of the printhead”. As evident from a curve 250 (FIG. 2B), the value of RON of the transistor increases over the life of the printhead till a predetermined value of life of printhead (marked as 252 in FIG. 2B). After this value of life of printhead, the value of RON of the transistor reaches a saturation value and remains substantially constant with increase in value of life of printhead.
Generally, an increase in the value of RON of transistors in a chip of a printhead of an ink-jet printer has an adverse effect on a printing efficiency of the printhead, thereby deteriorating a print quality thereof. More specifically, the increase in the value of RON of the transistors reduces the heating effect produced by the heaters of the printhead. Consequently, ink bubbles formed in the bubble chambers of the nozzle plate, due to the insufficient heating effect produced by the heaters, may not be uniform, thereby causing formation of poor quality ink dots on a printing medium. Accordingly, the printing efficiency of the printhead reduces over its life span.
Accordingly, there persists a need for a printhead of a printer, which overcomes the drawbacks and limitations of prior art printheads. Further, there persists a need for a method for improving a printing efficiency of a printhead over a life span thereof. More specifically, there persists a need for a method for compensating the increase in an ON resistance (RON) of transistors in a chip of a printhead over a life of the printhead.